1. Field of the Invention
The present invention is directed to an actively shielded planar gradient coil for pole plate magnets of the type employable in a magnetic resonance imaging apparatus.
2. Description of the Prior Art
Pole plate designs are usually utilized for nuclear magnetic resonance tomography systems in the low-field range that work with permanent magnets or normally conductive electromagnets. They are advantageous with respect to patient comfort and accessibility due to their open design.
U.S. Pat. No. 5,166,619, for example, discloses such a nuclear magnetic resonance tomography apparatus and is schematically shown in FIG. 1 herein for explaining a problem associated with these types of arrangements. In this embodiment of a nuclear magnetic resonance tomography apparatus having a basic field magnet 1 in the form of a C-magnet, the static, basic magnetic field proceeds parallel to the z-axis of a rectangular coordinate system having the axes x, y, z. This nuclear magnetic resonance tomography apparatus is provided for examination of a test subject, particularly a human body, whose physical axis extends in the direction of the x-axis of the coordinate system and whose body region to be examined is located in the imaging volume 2 between circular pole plates 3 and 4 of the basic field magnet 1. The basic magnetic field B.sub.o that proceeds in the direction of the z-axis of the coordinate system is generated by electrical coils 5 and 6. The origin of coordinates of the coordinate system, which is shown outside the imaging volume 2 only for clarity, should lie in the middle between the pole plates 3 and 4, so that the z-axis coincides with the rotational axis of the pole plates 3 and 4. The spacing H between the pole plates 3 and 4 can, for example, amount to 45 cm. The basic magnetic field B.sub.o is closed via a magnetic yoke 9.
Respective sub-coils 11 and 12 of the gradient coils, implemented as flat coils, are embedded in recesses of the pole plates 3 and 4. A separate pair of sub-coils is provided for each gradient direction x, y, z.
The interactions of the pulsed gradient coils with the various structures of the pole plate magnets are disproportionately more complicated than in the case of systems having a Helmholtz coil arrangement for the magnets. In addition to classic eddy currents, diffusion events occur and hysteresis effects in the pole shoes lead to transient noise fields that generally behave in a non-linear fashion and, moreover are dependent on the prior magnetic history of the system. These noise fields have a negative effect on the image quality. This is especially true for modern, fast imaging methods (for example, echo planar imaging) since gradients having a high amplitude are switched very quickly in such methods.
There are a number of proposals as to how these disturbing effects can be diminished. In general, the approach has been to try to device a specifically fashioned pole plate surface, to suppress the eddy currents as well as to conduct the stray flux of the gradient coil in a defined slice, having known behavior and comprised of a material with suitable permeability. For example, it is known to implement those parts of the pole plates facing toward the gradient coils as a wound iron tape having intervening insulating layers. Such measures, however, result in an adequate image quality only for comparatively simple sequence types (for example, spin echo sequence). Similar to the case of magnetic resonance systems with a cylindrical examination space, the disturbing interactions can be noticeably reduced by employing actively shielded gradient coils i.e., every gradient coil, or every sub-coil, is composed of a primary coil and a secondary coil lying parallel thereto. The primary coil and the secondary coil respectively have oppositely directed currents and are dimensioned such that the magnetic field of the gradient coil in the direction of the pole shoes is substantially fully compensated. The simple proposal of two coils arranged parallel to one another with opposite current flow, however, is technically impractical, as shall be set forth below with reference to FIGS. 2-5.
FIG. 2 shows the basic structure of a simple, actively shielded, transversal gradient coil (in this case, for the gradient in x-direction) in, perspective; the sectional view of FIG. 3 shows the corresponding course of the field lines. The gradient coil for the x-direction is composed of upper and lower primary coils 11a and 12a and lower and upper secondary coils 11b and 12b. The individual sub-coils are respectively constructed in conformity with U.S. Pat. No. 5,166,619.
When edge fringing is left out of consideration given this coil configuration and a limitation is initially made to the current density at x=0, then the following estimate for the flux .PHI. can be undertaken: EQU .PHI..about.(r.multidot.z.sub.p)/2.mu..sub.o .multidot.G.sub.x =H.sub.x .multidot..DELTA..sub.z
wherein r is the radius of the overall coil arrangement, Z.sub.p is the z primary coil, G.sub.x is the gradient in x-direction, H.sub.x is the field strength in x-direction and .DELTA.z is the spacing between the primary and secondary coils.
The current density J.sub.ys given x=0 at the location z.sub.s of the secondary coils 11b and 12b is calculated therefrom as: EQU J.sub.ys =-H.sub.x =-z.sub.p /.mu..sub.o .multidot.G.sub.x .multidot.(r/2.DELTA.z)
If the current density of an unshielded coil given x=0 is referenced J.sub.0 =z.sub.p /.mu..sub.0 .multidot.G.sub.x, then EQU J.sub.yp =J.sub.ys +J.sub.o
is valid for the required current density at the location z.sub.p of the primary coils 11a and 12a. The following estimates for the relationship of the required current densities derive therefrom: EQU J.sub.ys /J.sub.0 =-(r/2.DELTA.z) EQU J.sub.yp /J.sub.0 =1+(r/2.DELTA.z)
When typical numerical values are introduced for pole plate magnets (for example, R=0.48 m, .DELTA.z=0.03 m), then J.sub.ys /J.sub.0 =-8 and J.sub.yp /J.sub.0 =9 are obtained. In the aforementioned example, this means that approximately seven times the current density of an unshielded coil is required overall for an actively shielded gradient coil.
FIG. 4 shows a coil arrangement (unshielded) corresponding to the initially cited U.S. Pat. No. 5,166,619, whereby only half of a symmetrically constructed sub-coil is shown. One can clearly see that the turn density at the outer edge of the coil is to a multiple of the turn density in the inside of the coil, for example given x=0. It may be seen therefrom that a noticeable increase of the current density at a given current seems possible by increasing the turn density in the inner region of the coil. An increase in the turn density at the outer edge of the coil can only be achieved at the expense of (i.e., by decreasing) the conductor cross section.
The current density for an actively shielded gradient coil that is higher by a factor of 9 in the aforementioned example would, at a given current, require a number of turns higher by a factor of 9 or would assume a current higher by a factor of 9 with a given number of turns. The dissipative losses of the gradient coil increase noticeably both in the case of operation of the gradient coil with higher current as well as in the case of a higher number of turns with a necessarily smaller conductor cross section. In particular, extremely high losses locally occur in the outer region of the gradient coil.
FIG. 5 shows a coil design for an actively shielded gradient coil, namely for the primary coil 11a and the secondary coil 11b of one half of a symmetrically constructed sub-coil. Differing from the coil design of FIG. 2, FIG. 5 shows what is referred to as a "fingerprint" arrangement as is obtained, for example, in a method disclosed in U.S. Pat. No. 5,309,107. By comparison to the ill that is three times higher was selected, so that the number of turns is reduced by one-third with a given current density. One can see approximately 17 conductor elements in the central structure of the primary coil, these being mainly oriented in the z-direction. By contrast, the classic unshielded structure in FIG. 4 is composed of six such elements. When the coil current that is three times larger, is also taken into consideration and the current densities are correspondingly scaled, then the following relationship of the equivalent current densities of shielded and unshielded coil are obtained: ##EQU1##
The fact that a ratio of only 5.5 arises by comparison to the factor 9 in the earlier estimate is attributed to the advantages of an energy-minimizing design.
One can see a high density of the winding curve in the edge region of the coils in FIG. 5, this arising due to the high current density. High thermal loads occur in these edge regions. The technically required outlay for cooling such an arrangement has hitherto prevented practical solutions for actively shielded, transversal gradient coils in pole plate magnets.